diff --git a/README.org b/README.org
index 135f6df..4d5ba72 100644
--- a/README.org
+++ b/README.org
@@ -3,17 +3,19 @@
 [[https://github.com/syl20bnr/spacemacs][file:https://cdn.rawgit.com/syl20bnr/spacemacs/442d025779da2f62fc86c2082703697714db6514/assets/spacemacs-badge.svg]] [[https://melpa.org/#/winum][file:https://melpa.org/packages/winum-badge.svg]] [[https://stable.melpa.org/#/winum][file:https://stable.melpa.org/packages/winum-badge.svg]]
 
 * Contents :TOC:noexport:
- - [[#introduction][Introduction]]
- - [[#whats-changed-since-window-numbering][What's changed since window-numbering]]
- - [[#installation][Installation]]
-   - [[#using-melpa][Using Melpa]]
-   - [[#manual-installation][Manual installation]]
- - [[#how-to-use][How to use]]
- - [[#configuration][Configuration]]
-   - [[#keybindings][Keybindings]]
-   - [[#customize-options][Customize options]]
-   - [[#configuration-file-example][Configuration file example]]
- - [[#future-developments][Future developments]]
+- [[#introduction][Introduction]]
+- [[#changelog][Changelog]]
+  - [[#nov-15-2018][Nov. 15 2018]]
+  - [[#initial-release-whats-changed-since-window-numbering][Initial release: what's changed since window-numbering]]
+- [[#installation][Installation]]
+  - [[#using-melpa][Using Melpa]]
+  - [[#manual-installation][Manual installation]]
+- [[#how-to-use][How to use]]
+- [[#configuration][Configuration]]
+  - [[#keybindings][Keybindings]]
+  - [[#customize-options][Customize options]]
+  - [[#configuration-file-example][Configuration file example]]
+- [[#future-developments][Future developments]]
 
 * Introduction
 Window numbers for Emacs: Navigate your windows and frames using numbers !
@@ -25,7 +27,12 @@ from [[https://github.com/abo-abo/ace-window][ace-window]].
 This version brings, among other things, support for number sets across multiple
 frames, giving the user a smoother experience of multi-screen Emacs.
 
-* What's changed since window-numbering
+* Changelog
+** Nov. 15 2018
+- Added customize variable =winum-format=, a format string to configure how the
+  window number appears in the mode-line
+
+** Initial release: what's changed since window-numbering
 This package brings a lot of additions to the old window-numbering:
 
 - Number sets across multiple frames, giving a smoother experience of
@@ -241,6 +248,14 @@ Several options are available through Emacs' Customize interface under
 
   Default: =t=
 
+- =winum-format=
+
+  Format string defining how the window number looks like in the mode-line.
+  This string is passed to the =format= function along with the result of
+  =winum-get-number-string=.
+
+  Default: =" %s "=
+
 - =winum-mode-line-position=
 
   The position in the mode-line =winum-mode= displays the number.
@@ -297,6 +312,7 @@ available winum options.
         winum-auto-assign-0-to-minibuffer t
         winum-assign-func                 'my-winum-assign-func
         winum-auto-setup-mode-line        t
+        winum-format                      " %s "
         winum-mode-line-position          1
         winum-ignored-buffers             '(" *which-key*"))
 
@@ -321,9 +337,4 @@ available winum options.
 
 - Autocomplete read-from-minibuffer
 
-- Add =winum-format= customize variable
-
-  Format string for displaying the window number in the mode-line. Default to
-  " %n " to get a space on each side of the window number.
-
 - Things that you have thought of and I haven't :smile_cat:
diff --git a/winum.el b/winum.el
index ef22ffd..c130592 100644
--- a/winum.el
+++ b/winum.el
@@ -143,6 +143,13 @@ numbers in the mode-line."
   :group 'winum
   :type  'integer)
 
+(defcustom winum-format " %s "
+  "Format string defining how the window number looks like in the mode-line.
+This string is passed to the `format' function along with the
+result of `winum-get-number-string'."
+  :group 'winum
+  :type  'string)
+
 (defcustom winum-ignored-buffers '(" *which-key*")
   "List of buffers to ignore when assigning numbers."
   :group 'winum
@@ -208,6 +215,10 @@ To get a number given a window, use the `cdr' of a value.
 
 Such a structure allows for per-frame bidirectional fast access.")
 
+(defvar winum--mode-line-segment
+  '(:eval (format winum-format (winum-get-number-string)))
+  "What is pushed into `mode-line-format' when setting it up automatically.")
+
 (defvar winum--last-used-scope winum-scope
   "Tracks the last used `winum-scope'.
 Needed to detect scope changes at runtime.")
@@ -404,8 +415,6 @@ WINDOW: if specified, the window of which we want to know the number.
   (remove-hook 'window-configuration-change-hook 'winum--update)
   (setq winum--frames-table nil))
 
-(defvar winum--mode-line-segment '(:eval (winum-get-number-string)))
-
 (defun winum--install-mode-line (&optional position)
   "Install the window number from `winum-mode' to the mode-line.
 POSITION: position in the mode-line."